The following paragraphs are not an admission that any of the information below is common general knowledge or citable as prior art.
Selenium is an essential trace element, but becomes toxic at very low concentrations. Selenium accumulates in the bodies of plants and fish that live in selenium-contaminated water and in the bodies of wildlife and people that eat those plants and fish. In people, elevated selenium concentrations may cause neurological damage and hair and nail loss.
Selenium has been treated in biological reactors, for example as described in U.S. Pat. No. 6,183,644 and International Publication Number WO 2007/012181, and as used in ABMet™ reactors sold by the GE Water and Process Technologies. In such reactors, dissolved selenium is removed from contaminated water by treating the water in a reactor containing selected endemic and other selenium reducing organisms. Microbes may be isolated from the specific water or imported from other selenium contaminated water. The microbes are then screened for ability to reduce selenium under the site specific environmental conditions. The selected microbes are optimized for selenium reduction, then established in a high density biofilm within a reactor. The selenium contaminated water is passed through the reactor with optimized nutrient mix added as needed. The elemental selenium is precipitated and removed from the water. The entirety of U.S. Pat. No. 6,183,644 is incorporated herein by this reference to it.
Further, U.S. Publication No. 2010/0300963 provides for an apparatus or process for treating flue gas desulfurization blow down water or other wastewaters having selenium or nitrate or both, or a process or apparatus for biologically removing inorganic contaminants, for example nitrogen, selenium, arsenic, mercury or sulphur, from wastewater. The entirety of U.S. Publication No. 2010/0300963 is incorporated herein by this reference to it.
A prior art biological reactor, such as ABMet, is schematically illustrated in FIG. 1. A reactor 100 comprises a media bed 101 supporting a population of selenium-reducing microorganisms. Activated carbon may be employed as the medium and is typically provided in the form of granular activated carbon (GAC). Wastewater enters the reactor 100 through upper port 106, flows downward through the media bed 101 and exits through the lower port 102. As solids accumulate within the media bed 101, the pressure drop across the media bed 101 will increase. At a selected time interval or pressure point, the media bed may be flushed. This operation may be accomplished by utilizing backwash port 103 and its associated distribution and collection system 105. The upflow velocity applied during flushing results in an upward expansion of the media bed into a bed expansion layer. The backwash water and flushed entrained solids may be removed through trough 108. The reactor 100 provides an expansion space above the media bed 101 to accommodate media expansion. A headspace layer is provided above the expansion layer and below reactor cover 109 to accommodate gases released by microorganisms.